In recent years, semiconductor devices having a trench gate structure have been focused on. While a channel region is formed on the surface of a semiconductor layer in a semiconductor device having a planar gate structure, a channel region is formed on the sidewall surface of a trench provided in a semiconductor layer in a semiconductor device having the trench gate structure. Thus, in a semiconductor device having the trench gate structure, miniaturization and reduction in on-resistance can be expected more than in a semiconductor device having the planar gate structure. Therefore, semiconductor devices having the trench gate structure are being developed particularly in the field of power devices.
Specifically, since miniaturization is not limited by the effect of junction field effect transistors (JFETs), a semiconductor device having the trench gate structure has the advantages of miniaturizing a trench, and reducing on-resistance and switching loss.
However, semiconductor devices having the trench gate structure have the problem of increasing gate resistance as a result of reduction in the cross-sectional area of a gate electrode with the miniaturization of a trench.
In order to address the problem, extending a gate electrode to the periphery of a trench, i.e., forming a T-shaped gate electrode is considered to mitigate an increase in gate resistance (see, for example, Patent Document 1).
In a semiconductor device having the trench gate structure, forming an appropriate gate insulating film in a trench is important. Specifically, the thickness of the gate insulating film needs to be reduced on the sidewall surface of the trench, in which a channel region is formed, to reduce the threshold voltage at switching, and the thickness of the gate insulating film needs to be increased at the bottom of the trench to avoid electric field concentration.
However, if the thickness of the entire gate insulating film is increased to increase the breakdown electric field, the threshold voltage at switching increases. On the other hand, if the thickness of the entire gate insulating film is reduced to lower the threshold voltage at switching, the electric field concentration occurs at the bottom of the trench.
Then, for example, Patent Document 2 suggests forming a thicker gate insulating film at the bottom of a trench using the difference in the orientation between the sidewall surface and the bottom of the trench. For example, Patent Document 3 suggests selectively forming a mask on the sidewall surface of a trench in forming a gate insulating film, thereby preventing formation of an oxide film on the sidewall surface of the trench and forming the thick gate insulating film on the portions other than the sidewall surface of the trench.